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Effectiveness of Prescribed Fire as a Fuel
Treatment in Californian Coniferous Forests
Nicole M. Vaillant1, JoAnn Fites-Kaufman2, Scott L. Stephens3
Abstract—Effective fire suppression for the past century has altered forest structure and
increased fuel loads. Prescribed fire as a fuels treatment can reduce wildfire size and
severity. This study investigates how prescribed fire affects fuel loads, forest structure,
potential fire behavior, and modeled tree mortality at 80th, 90th, and 97.5th percentile
fire weather conditions on eight National Forests in California. Potential fire behavior
and effects were modeled using Fuel Management Analyst. Prescription burning did not
significantly change forest structure at most sites. Total fuel loads (litter, duff, 1, 10, 100,
and 1000-hour) were reduced by 23 to 78 percent across the sites. This reduction in
fuels altered potential fire behavior by reducing rate of spread, flame length, and fireline
intensity. Increased torching index values coupled with decreased fuel loads reduced
crown fire potential post-treatment in some stands. Predicted tree mortality decreased
post-treatment as an effect of reduced potential fire behavior and fuel loads. With the
vast forested areas classified at high risk for catastrophic wildland fire in California, it
is most efficient to target stands that benefit the most from treatment.
Introduction
In many coniferous forests, fi re suppression has lead to higher tree densities
(Biswell 1959), changes in species composition (Weaver 1943), and higher
fuel loads (Dodge 1972), which have altered fi re regimes (Beaty and Taylor
2001; Stephens and Collins 2004). A recent analysis of fi re cause and extent
on U.S. Forest Service (USFS) lands from 1940 to 2000 demonstrated that
California experienced a significant increase in the total number of fi res
and had the most area burned relative to other regions in the United States
(Stephens 2005). Although the area burned has not significantly increased
from 1940 to 2000 in California (Stephens 2005), the wildland fi re problem
has only worsened as suppression has become more effective (Brown and
Arno 1991).
Fuels treatments can be effective at reducing the severity (Pollet and
Omi 2002; Agee and Skinner 2005; Finney and others 2005) and size of
wildland fi res (Stephens 1998; Piñol and others 2005). Reduction of surface
fuels, and in some cases crown fuels, can reduce the likelihood of crown fi res
(van Wagner, 1977). Typically, mechanical methods are used to alter stand
structure (i.e., reduce tree density, decrease basal area, increase the height to
live crown base, and reduce canopy cover) (Keyes and O’Hara 2002; Pollet
and Omi, 2002; Stephens and Moghaddas, 2005a,b). Prescribed fi re alone
can decrease surface and ladder fuels which reduce potential fi re behavior
and thus lower the risk of crown fi re and spot fi re ignition (van Wagtendonk
1996; Stephens 1998).
USDA Forest Service Proceedings RMRS-P-41. 2006.
In: Andrews, Patricia L.; Butler, Bret W.,
comps. 2006. Fuels Management—How to
Measure Success: Conference Proceedings.
28-30 March 2006; Portland, OR.
Proceedings RMRS-P-41. Fort Collins,
CO: U.S. Department of Agriculture,
Forest Service, Rocky Mountain Research
Station.
1
Graduate student at UC Berkeley and
a fi re ecologist for AMSET,Division
of Ecosystem Science, Department of
Environmental Science, Policy, and
Management, University of California,
Berkeley, Berkeley, CA.
[email protected]
2 Fire
ecologist for Nevada City, CA.
3 Assistant professor at UC Berkeley,
Berkeley, CA.
465
Vaillant, Fites-Kaufman, Stephens
Effectiveness of Prescribed Fire as a Fuel Treatment in Californian Coniferous Forests
The objective of this study is to determine how prescribed fi re effects fuel
loads, vegetation structure, and potential fi re behavior and effects in eight
National Forests in California. The null hypothesis investigated is that there
will be no significant difference in vegetation structure, fuel load, fi re behavior, and predicted tree mortality at each study site when comparing pre- and
post-treatment characteristics. Information from this study could be used to
assist in the development of forest management plans that use prescribed fi re
to reduce fi re hazards.
Methods
Study Location
Nine project sites are located on eight National Forests: the Klamath (one
on the eastern section, KNF E, and one on the western section, KNF W),
Lassen (LNF), Los Padres (LPF), Modoc (MDF), Mendocino (MNF), Plumas
(PNF), Shasta-Trinity (SHF) and Sierra (SNF) (fig. 1). LPF, MDF, MNF,
and SNF are dominated by yellow pine [>80% of basal area is composed of
ponderosa pine (Pinus ponderosa Laws) or Jeffrey pine (Pinus jeffreyi Grev.)]
and KNF E, KNF W, PNF, and SHF are in mixed-conifer forests.
Figure 1—Location of study sites.
466
USDA Forest Service Proceedings RMRS-P-41. 2006.
Effectiveness of Prescribed Fire as a Fuel Treatment in Californian Coniferous Forests
Vaillant, Fites-Kaufman, Stephens
Climate in the study sites is Mediterranean with a summer drought period
that extends into the fall. The majority of precipitation occurs during winter
and spring. Tree species present include ponderosa pine, Jeffrey pine, sugar
pine (Pinus lambertiana Dougl.), white fi r (Abies concolor Gord. and Glend.),
Douglas-fi r (Pseudotsuga menziesii (Mirb.) Franco), incense-cedar (Calocedrus
decurrens Torr.), western juniper (Juniperus occidentalis Hook.), California
black oak (Quercus kelloggii Newb.), canyon live oak (Quercus chrysolepis
Liebm.), and bigleaf maple (Acer macrophyllum Pursh).
The average elevation of the study sites ranges from approximately 1000 to
1600 m. Average slopes vary from three to 61 percent. Pre-treatment percentcover of tree canopy, shrubs, and grasses varies between study locations.
Treatments
All of the study sites were treated with prescribed fi re. The primary objectives of the prescribed burns were to reduce the potential for catastrophic
stand replacing fi re events and to reintroduce fi re into the ecosystem. Each of
the National Forests implemented their own prescribed fi res. The prescribed
fi res occurred either in spring or fall depending on weather, available personnel, and funding, with the majority of prescribed fi res taking place in the
spring (six out of nine).
Vegetation Measurements
In each of the nine project sites, vegetation was measured using 0.2 ha
randomly-placed, permanently-marked circular plots (26 total plots). Tree
information was collected in two nested subplots; 0.1-ha for all trees greater
than 15 cm diameter at breast height (d.b.h.), and 0.025 ha for trees 2.5 to
15 cm d.b.h. Tree measurements (species, d.b.h., height, height to live crown
base (HTLCB), and tree crown position (dominant, codominant, intermediate or suppressed)) are recorded for live trees; for snags species, d.b.h., and
total height was recorded. Canopy cover was measured every meter along
two perpendicular 50 m transects using a Moosehorn sight tube (Gill and
others 2000). Shrub measurements were also taken along the same transects
in each of the plots to estimate percent shrub cover. An ocular estimate of
percent cover by grasses was made along the shrub transect in a 1 m 2 frame
every 10 m.
Ground and Surface Fuel Characteristics
Surface and ground fuels were measured with four transects in each of
the plots using the line-intercept method (van Wagner 1968; Brown 1974).
For each transect, one-hour (0 to 0.64 cm diameter) and 10-hour (0.64
to 2.54 cm diameter) fuels were sampled from 0 to 1.83 m, 100-hour fuels
(2.54 to 7.62 cm diameter) from 0 to 3.66 m, and 1000-hour fuels (diameter
>7.62 cm) from 0 to 15.24 m. Species, diameter, and decay status (rotten or
sound) were recorded for all 1000-hour fuels. Litter, duff, and fuel bed depth
(cm) measurements were taken every 1.52 m totaling 10 per transect. Surface
and ground fuel loads were calculated using arithmetically-weighted coefficients specific to the California tree species based on the average basal area
fraction of the individual sites (van Wagtendonk and others 1998; Stephens
and Moghaddas 2005a).
USDA Forest Service Proceedings RMRS-P-41. 2006.
467
Vaillant, Fites-Kaufman, Stephens
Effectiveness of Prescribed Fire as a Fuel Treatment in Californian Coniferous Forests
Fire Modeling
Fire behavior and effects were modeled under upper 80th, 90th, and 97.5th
percentile fi re weather conditions. Eightieth, 90th, and 97.5th percentile fi re
weather represent moderate, high, and extreme fi re weather, respectively.
Percentile weather was computed using Fire Family Plus (Main and others
1990). Forty-three years (1961 to 2004) of weather data from the most
representative Remote Automated Weather Station (R AWS) for each site
(NFAM 2004) were analyzed to determine percentile weather conditions.
Fuels Management Analyst (FMA) was used to model fi re behavior and
effects (rate of spread, flame length, fi reline intensity, crowning index, torching index, and tree mortality) (Carlton 2005). Fire behavior predictions were
made for stand and fuel structures before and after prescribed burning. A
surface fuel model was assigned to each sampling plot based on stand structure, shrub cover, grass cover, and fuel loads (Scott and Burgan 2005).
Data Analysis
Paired t-tests were used to determine if significant differences (p<0.1)
existed in vegetation (trees ha–1, basal area ha–1, tree height, HTLCB, canopy
cover, crown bulk density (CBD)) and fuel loads (litter, duff, 1-hr, 10-hr, 100-hr,
1000-hr sound, 1000-hr rotten, total fuel load (1 to 1000-hr, litter and duff),
and fuel depth) for each site pre- and post-prescribed fi re (Zar 1999). The
choice of p<0.1 was made due to high natural variation found between plots
in each study site. The number of sample plots varied by site location due to
the ability of the individual National Forests to burn the proposed units and
because some prescribed fi res did not burn the entire intended area.
Results
Forest Structure
The inventory plots in the nine study locations included 860 live trees
greater than 2.5 cm d.b.h. pre-treatment and 801 post-treatment. No significant differences were found for any of the measured variables (basal area,
trees ha–1, d.b.h., tree height, HTLCB, canopy cover, CBD) at KNF W, MDF,
SHF or SNF (table 1). At LNF, LPF, MNF and PNF some but not all of the
variables were significantly different (table 1). All variables were significantly
different at KNF E except HTLCB.
Fuels Characteristics
A total of 104 fuel transects were analyzed over the nine project sites to
characterize surface and ground fuels pre- and post-prescribed burning. All
locations had a significant difference post-treatment in at least one of the fuels
parameters (table 2). All of the locations except PNF experienced a significant
reduction in litter loads. Total fuel load was reduced at all sites; however, the
difference was only significant at MNF and LPF.
Potential Fire Behavior
Rate of spread (ROS) increased for all sites with increasing percentile
weather (table 3). Post-treatment ROS either decreased or experienced no
change when compared to pre-treatment. Flame length (FL) increased with
468
USDA Forest Service Proceedings RMRS-P-41. 2006.
USDA Forest Service Proceedings RMRS-P-41. 2006.
Post
36.0a
42.6
48.8
27.3
24.3
27.2
35.9
33.8
40.8
Pre
37.0a
48.2
51.9
28.1
26.9
27.3
38.1
34.4
40.7
Basal area
(m2 ha–1)
585.0
490.0a
600.0
313.3
520.0
423.3
163.3
525.0
706.7a
Pre
420.0
405.0a
306.7
263.3
516.0
360.0
120.0
525.0
533.3a
Post
Trees (ha–1)
33.7
34.7
27.3
33.3
27.3
33.9
52.5
36.2
27.2a
Pre
34.5
36.2
37.1
34.1
26.6
35.3
58.6
36.2
29.6a
Post
DBH (cm)
Pre
2.2
7.4
9.2
10.0a
5.2
14.8
9.0
6.9a
8.5
Post
Post
4.4a
3.0a
3.5a
1.7a
3.8a
3.0a
10.0
1.9a
2.7a
Pre
11.1a
18.5a
18.9a
4.4a
5.6a
12.9a
4.3
5.4a
12.1a
Litter
different pre- versus post-treatment .
21.4
33.2
17.0
22.3a
13.6
16.7
22.5
28.9a
15.4
a=significantly
KNF E
KNF W
LNF
LPF
MDF
MNF
PNF
SHF
SNF
Duff
1.5
1.4
2.1
0.6
0.5a
0.3
1.3
0.9a
0.9
Pre
1-hr
0.9
0.3
0.6
0.2
0.2a
0.4
0.6
0.2a
0.5
Post
2.7
6.1
7.7
1.0
1.2
2.5
2.0
3.4a
2.3
Pre
1.3
1.4
3.8
0.8
0.9
1.6
2.1
1.0a
0.8
Post
10-hr
Table 2—Average fuel loads (metric t ha–1) pre- and post-treatment by site location.
Site
15.0
21.0
14.3
14.3
14.0
19.3a
27.9
19.1
14.4a
Pre
17.1
21.4
13.3
15.1
14.0
21.0a
31.4
19.1
15.6a
Post
Tree height
(m)
2.8
5.9
4.5
2.8
1.7a
3.6
5.1
7.6
5.3
Pre
2.5
1.3
2.7
4.1
2.8a
4.8
3.9
3.1
3.7
Post
100-hr
4.9
3.4
8.9
3.6
3.8
4.2
7.8a
10.4
7.1
Pre
3.5
9.7
7.4
0.0
1.2
2.7
13.8
0.4a
1.5
6.5
6.9
0.8
4.6
0.0
7.3
0.2
9.5a
2.5
Post
39.7
61.4
7.4
13.2
12.0
64.7a
25.2
17.6
6.0
Pre
76.3
96.6
24.0a
29.4
69.0a
64.7
30.5
51.0
43.2a
Pre
0.0
33.7
29.8
0.0
3.6
12.2a
15.3
7.6
2.6
82.7
136.0
65.1
44.3a
35.6
103.5a
74.2a
64.3
43.6
Pre
17.8
53.9
50.3
21.3a
16.5
44.1a
41.0a
30.2
21.3
Post
1-1000-h plus
litter, duff
70.6
93.1
19.8a
30.2
50.7a
62.1
27.6
44.2
35.3a
Post
Canopy cover
(percent)
Post
1000-hr
rotten
4.6
8.9
9.4
3.2
4.8
4.2
11.3a
11.7
7.1
Post
HTLCB (m)
1000-hr
sound
Pre
HTLCB= height to live crown base, CBD= crown bulk density, a=significantly different pre- versus post-treatment.
KNF E
KNF W
LNF
LPF
MDF
MNF
PNF
SHF
SNF
Site
0.091a
0.050
0.044
0.044a
0.050
0.089
0.067
0.033
0.071
Post
28.2a
25.6
14.7
9.7
7.9
67.0a
16.2a
11.1a
51.3
Pre
7.7a
12.3
11.5
3.4
3.5
8.3a
10.3a
7.4a
8.1
Post
Fuel
depth (cm)
0.054
0.046
0.049a
0.057
0.090
0.069
0.034
0.074
0.094a
Pre
CBD (kg m–3)
Table 1—Average pre- and post-treatment vegetation structure for all trees greater than 2.5 cm d.b.h. by site location for nine stands in eight Californian National Forests.
Effectiveness of Prescribed Fire as a Fuel Treatment in Californian Coniferous Forests
Vaillant, Fites-Kaufman, Stephens
469
470
0.7
0.5
0.5
0.7
0.4
2.4
1.5
0.4
0.6
0.9
0.7
0.7
0.7
0.5
2.6
1.7
0.5
0.7
15.4
0.9
0.8
4.6
0.7
3.9
2.0
0.6
0.7
15.7
24.9
2.1
12.5
2.9
16.0
1.8
0.7
5.4
0.4
0.3
0.3
0.4
0.2
1.2
0.6
0.2
0.3
0.6
0.9
0.7
1.4
0.8
3.2
0.6
0.3
0.9
0.4
0.3
0.3
0.4
0.3
1.2
0.7
0.3
0.3
0.7
4.3
0.8
1.5
1.0
3.7
0.7
0.4
1.4
6.3
0.3
0.3
0.5
0.3
1.5
0.7
0.3
0.3
6.6
5.2
0.9
2.8
1.2
7.1
0.7
0.4
1.5
FI
90th
97.5th
33.9
15.6
15.9
31.2
12.1
408.4
94.4
12.1
17.7
42.6 46828.1
19.7
24.2
19.9
21.9
33.9
53.4
15.9
24.2
451.3
659.7
107.2
125.4
14.5
19.7
21.9
21.9
75.5
153
49286.6
213.3 10476.3 16751.5
160.3
201.8
248.6
570.8
685.4
2950.5
185.5
245.9
379.3
5430.0 8500.2 44942.9
100.8
111.3
129.9
25.3
30.2
40.4
22.9
578.4
685.9
- - - - - - (kW m–1) - - - - - -
80th
ROS-rate of spread; FL-flame length; FI-fireline intensity; TI-torching index; CI-crowning index.
KNF E
KNF W
LNF
LPF
MDF
MNF
PNF
SHF
SNF
Post
2.9 7.2
2.0 12.6
1.4 1.8
3.6 4.6
1.2 1.6
6.4 8.3
1.5 1.6
0.4 0.5
1.8 4.5
- - - - (m) - - - -
- - (m min–1) - -
Pre
KNF E
KNF W
LNF
LPF
MDF
MNF
PNF
SHF
SNF
FL
80th 90th 97.5th
ROS
80th 90th 97.5th
Site
TI
90th
97.5th
434.7
3023.2
2167.4
299.6
619.0
104.7
981.9
2677.8
1299.2
152.8
76.1
595.2
35.3
135.9
48.3
770.3
1894.2
212.2
347.0
2422.0
1766.4
238.2
521.8
83.2
782.0
2134.9
1052.0
119.5
26.9
462.7
26.2
104.5
37.6
612.8
1491.6
140.1
321.2
2218.5
1650.3
217.8
466.9
76.2
715.1
2012.2
968.1
109.7
24.0
417.3
23.2
88.5
34.2
560.1
1401.0
117.7
- - - - - - (km h–1) - - - - - -
80th
Table 3—Average modeled fire behavior under 80th, 90th, and 97.5th percentile weather by site location.
CI
90th
97.5th
42.8
58.0
61.8
84.6
85.6
34.3
51.2
82.9
47.2
41.5
51.6
60.6
75.9
73.1
34.1
44.2
80.6
45.6
38.4
53.0
57.2
74.8
85.9
31.6
45.1
77.2
40.4
37.2
47.0
56.0
67.0
68.4
31.4
38.8
75.1
29.8
34.2
46.8
52.7
67.8
71.2
28.8
38.1
72.4
38.1
33.2
41.3
51.4
60.6
60.8
28.6
32.5
70.4
24.7
- - - - (km h–1) - - - -
80th
Vaillant, Fites-Kaufman, Stephens
Effectiveness of Prescribed Fire as a Fuel Treatment in Californian Coniferous Forests
USDA Forest Service Proceedings RMRS-P-41. 2006.
Effectiveness of Prescribed Fire as a Fuel Treatment in Californian Coniferous Forests
Vaillant, Fites-Kaufman, Stephens
respect to higher percentile fi re weather pre-treatment except at PNF and
SHF where no change occurred between the 90th and 97.5th percentiles
(table 3). FL was shorter post-treatment as compared to pre-treatment in all
locations except PNF where it did not change. Modeled fi reline intensity (FI)
increased as percentile weather increased both pre- and post-treatment for
all site locations except SNF (table 3). FI decreased post-treatment as compared to pre-treatment for all site locations. Torching index (TI) decreased
as percentile weather increased pre- and post-treatment (table 3). Crowning
index (CI) decreased with increasing percentile weather except at MDF where
it only increased between the 80th and 90th percentile. CI increased slightly
post-treatment for all locations, following the decreasing trend with respect
to increasing severity of fi re weather.
Fire type (FT) remained 100 percent surface fi re in the LNF, PNF, SHF,
and SNF sites pre- and post-treatment for all weather scenarios (table 4).
Prescribed fi re changed predicted FT in the KNF E, KNF W, LPF, MDF,
and MNF sites by either decreasing the likelihood of crown fi re or decreasing
the severity of crown fi re. At 80th and 90th percentile fi re weather conditions,
all post-treatment sites experienced only surface fi re.
Predicted Tree Mortality
Probability of mortality was modeled for four diameter classes (2.5 to 25,
25 to 51, 51 to 76, >76 cm d.b.h.) as well as for all trees at each study site
pre- and post-treatment (table 5). For all sites, a higher percentage of trees was
predicted to die prior to treatment than after treatment. A higher amount of
Table 4—Modeled fire type under 80th, 90th, and 97.5th percentile weather by site location.
Site
80th
90th
97.5th
33%PCF,66%SF
33%PCF,66%SF
100%SF
33%PCF,66%SF
100%SF
40%PCF, 60%SF
100%SF
100%SF
100%SF
33%PCF,66%SF
33%SF, 66%PCF
100%SF
33%PCF,66%SF
100%SF
40%PCF, 60%SF
100%SF
100%SF
100%SF
33%ACFWD, 66%SF
33%SF,33%PCF,33%ACFPD
100%SF
33%SF, 66%PCF
33%PCF, 66%SF
20%PCF, 20%ACFPD, 60%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
100%SF
33%ACFWD, 66%SF
100%SF
100%SF
33%PCF, 66%SF
100%SF
100%SF
100%SF
100%SF
100%SF
Pre
KNF E
KNF W
LNF
LPF
MDF
MNF
PNF
SHF
SNF
Post
KNF E
KNF W
LNF
LPF
MDF
MNF
PNF
SHF
SNF
SF=surface fire; PCF=passive crown fire; ACFWD=active crown fire wind driven; ACFPD=active crown fire plume
dominated.
USDA Forest Service Proceedings RMRS-P-41. 2006.
471
Vaillant, Fites-Kaufman, Stephens
Effectiveness of Prescribed Fire as a Fuel Treatment in Californian Coniferous Forests
mortality was predicted in smaller diameter classes (2.5 to 25 cm and 25 to
51 cm d.b.h.) regardless of location, weather condition, or treatment status.
An increase in mortality with respect to increasing predicted fi re weather
conditions occurred in most study sites prior to prescribed fi re; the trend was
not the same post-treatment (table 5).
Table 5—Average pre- and post-prescribed burn percent predicted mortality by diameter class and
site location for three percentile weather conditions.
DBH
range
(cm)
KNF E KNF W
LNF
LPF
MDF
MNF
PNF
SHF
SNF
Pre
80 th
2.5-25
25-51
51-76
>76
All
62.7
21.9
6.9
•
30.5
90.6
52.1
8.0
4.6
38.8
56.1
24.1
7.3
2.0
22.4
99.4
70.5
6.0
2.8
44.7
86.0
27.5
8.4
•
40.6
95.4
79.6
•
•
87.5
64.3
17.5
6.4
2.0
22.5
65.5
20.4
5.2
3.6
23.7
58.7
17.3
4.9
2.0
20.7
90 th
2.5-25
25-51
51-76
>76
All
66.5
26.7
8.2
•
33.8
98.1
84.1
50.0
48.3
70.1
57.8
25.0
8.0
2.0
23.2
99.6
79.6
8.9
3.8
48.0
92.4
32.6
12.1
•
45.7
96.9
85.7
•
•
91.3
64.7
17.5
6.4
2.0
22.6
65.5
20.4
5.2
3.6
23.7
77.5
28.8
5.4
2.0
28.4
97.5th
2.5-25
25-51
51-76
>76
All
69.5
46.6
35.6
•
50.6
99.1
87.9
64.0
58.6
77.4
59.2
26.6
9.3
2.0
24.3
99.6
89.0
39.4
5.5
58.4
97.8
44.0
20.6
•
54.1
99.2
95.5
•
•
97.3
66.0
17.6
6.4
2.0
23.0
65.5
20.4
5.2
3.6
23.7
83.8
37.2
6.5
2.0
32.4
80 th
2.5-25
25-51
51-76
>76
All
52.3
21.1
6.9
•
26.8
58.9
23.1
5.6
2.9
22.6
46.0
23.2
8.6
2.0
19.9
52.4
22.5
7.1
2.4
18.3
53.6
17.2
5.0
•
25.2
81.7
58.8
•
•
70.2
52.2
17.8
6.3
2.0
19.6
40.2
18.6
5.2
3.6
16.9
48.0
13.9
4.3
2.0
17.1
90 th
2.5-25
25-51
51-76
>76
All
52.3
21.1
6.9
•
26.8
58.9
23.1
5.6
2.9
22.6
46.0
23.2
8.6
2.0
19.9
52.4
22.5
7.1
2.4
18.3
53.6
17.2
5.0
•
25.2
85.7
67.2
•
•
76.5
52.2
17.8
6.3
2.0
19.6
40.2
18.6
5.2
3.6
16.9
48.0
13.9
4.3
2.0
17.1
97.5th
2.5-25
25-51
51-76
>76
All
65.7
46.7
35.6
•
49.3
58.9
23.1
5.6
2.9
22.6
46.0
23.2
8.6
2.0
19.9
56.9
31.0
16.6
2.4
18.8
53.9
17.9
5.2
•
25.7
87.6
78.0
•
•
82.8
52.3
17.8
6.3
2.0
19.6
40.2
18.6
5.2
3.6
16.9
48.0
13.9
4.3
2.0
17.1
Post
• = no trees in this diameter class for this location.
472
USDA Forest Service Proceedings RMRS-P-41. 2006.
Effectiveness of Prescribed Fire as a Fuel Treatment in Californian Coniferous Forests
Vaillant, Fites-Kaufman, Stephens
Discussion
Topography, weather, and fuels all play a role in the hazard and severity of
wildland fi re. Altering the fuel load is the most feasible and important factor
to decrease hazard and severity of wildland fi re. The vertical and horizontal
continuity of surface fuels (litter and downed woody debris), ladder fuels (shrubs
and small trees), and/or canopy fuels (large trees) must be broken to reduce
fi re severity. Reduction in surface fuels can reduce FI, increasing HTLCB can
reduce the risk of torching, and reduction in crown density can limit tree-totree spread of crown fi res (Agee 2002; Hessberg and Agee 2003; Agee and
Skinner 2005).
Many studies in ponderosa pine and mixed-conifer forests document the
effectiveness of prescribed fi re in reducing future fi re severity (Weaver 1943;
Biswell and others 1973; Kauffman and Martin 1989; van Wagtendonk 1996;
Stephens 1998; Miller and Urban 2000; Pollet and Omi 2002; Finney and
others 2005; Knapp and others 2005; Stephens and Moghaddas 2005a,b).
Prescribed fi re effectively reduces surface fuel loads as well as kills shrubs and
small diameter trees which reduce ladder fuels. Understory burning can also
raise the height to live crown base through scorching of lower branches. One
unifying goal of the prescribed burns analyzed in this work was to reduce
the risk of stand-replacing catastrophic fi re.
Stand characteristics did not significantly change in four of the nine site locations after treatment. This is consistent with many of the studies mentioned
above. However, KNF E did experience a significant change in basal area, trees
ha–1, d.b.h., tree height, canopy cover, and CBD post-prescribed fi re. This may
be partially due to a tree blowdown event between plot readings (Kit Jacoby,
personal communication). In the rest of the sites there were few differences
in stand structure pre- and post-treatment. TI and CI moderately increased at
all sites post-treatment, which indicates the need for an increase in wind speed
to initiate and maintain crown fi re. Overall, the modeled outputs document a
reduced percentage of crown fi res post-treatment; five treatments had a component of passive crown fi re pre-treatment and two post-treatment (table 4).
If the primary goal of the prescribed fi re treatment is to reduce the potential
of stand replacing catastrophic wildfi res, then TI and CI might be of particular
interest. CI only increased slightly for all sites post-treatment indicating that
the prescribed fi re treatments did not effect the overstory (CBD or tree canopy
cover). Under the 80th percentile fi re weather condition, the untreated sites
are unlikely to initiate crown fi re due to high TI (table 3). For the 90th and
97.5th percentile fi re weather conditions, pre-treatment values of TI and CI
make the KNF W, LPF, and MNF sites more vulnerable to active crown fi re
(table 3). The reduction in likelihood of crown fi re is due to a combination
of changes in stand structures and surface fuel loads. Crown fi re is not solely
linked to canopy characteristics; surface fuel loads also play a critical role in
active crown fi re initiation and spread. If surface fi reline intensity exceeds
the critical level needed to initiate an active crown fi re, the canopy is likely
to burn as long as high surface fuel loads are present.
Fuel bed depth was significantly reduced at the KNF E, MNF, PNF and
SHF sites; however, fuel bed depth was reduced by at least 20 percent at
the remaining five sites, but was not statistically significant. Total fuel loads
(surface and ground) were reduced significantly at LPF, MNF and PNF.
The relatively high consumption of ground and surface fuels is consistent
with past studies (Kilgore and Sando 1975; Kauffman and Martin 1989;
Stephens and Finney 2002; Knapp and others 2005). Prescribed fi re without
USDA Forest Service Proceedings RMRS-P-41. 2006.
473
Vaillant, Fites-Kaufman, Stephens
Effectiveness of Prescribed Fire as a Fuel Treatment in Californian Coniferous Forests
crown thinning has been shown to greatly reduce fi reline intensity relative
to no treatment (van Wagtendonk 1996; Stephens 1998). A reduction in
surface fuel loads generally results in decreased fi re severity, ROS, FL, and
FI. Altered stand structures also contributed to the increase of surface fi res
versus crown fi res post-treatment. Smaller diameter trees killed by prescribed
fi re are initially standing dead fuel. Eventually these trees will fall and contribute to the surface fuel loads (Stephens 1998; Agee 2003), necessitating
future prescribed fi res to keep hazards low.
Predicted tree mortality was higher pre-treatment than post-treatment for
all locations under low, moderate, and extreme fi re weather. Probability of tree
mortality is primarily based on percent crown scorched which is derived from
crown ratio, species tree height, and tree diameter (Reinhardt and others
1997). Predicted tree mortality was greatest in the smallest diameter class
(2.5 to 25 cm d.b.h.) and decreased with increasing diameter classes (table 5).
Increases in percentile fi re weather post-treatment did not increase the likelihood of overall tree mortality at five sites (KNF W, LNF, PNF, SHF, SNF), it
only slightly increased tree mortality in two sites (LPF and MDF), and it greatly
increased tree mortality in two sites (KNF E and MNF). Predicted mortality almost doubled for all diameters at KNF E between the 90th and 97.5th
percentile conditions post-treatment where fi re type also changed; however,
mortality was still lower relative to pre-treatment conditions (tables 4 and 5).
If reduction of potential stand replacing fi res is the primary goal of prescribed fi re treatments, selection of treatment locations must consider the
existing fi re hazards. Four of the nine study sites examined here only experienced modeled surface fi re in pre-treatment conditions, including extreme
fi re weather conditions (table 4). Post-treatment potential fi re behavior (ROS,
FL, FI) was reduced, but these stands were not at risk of crown fi re before
treatment. On the other hand, three of the nine sites were at an elevated
risk of crown fi re (low TI and CI) pre-treatment at 97.5th percentile weather
conditions (table 3). For the sites that would experience only surface fi re,
treatment is not warranted based on the reduction of potential fi re behavior
and effects. Sites experiencing low TI and CI values may benefit from a mechanical treatment (such as thinning from below) prior to prescribed fi re to
further reduce the risk of active crown fi re.
In addition to the reduced potential for stand replacing catastrophic wildland fi res, reintroduction of fi re into the ecosystem was a primary goal of
these prescribed fi re treatments. Seasonality of prescribed fi re is important
from an ecological and fuels consumption standpoint. Fire history data from
the southern Cascades in California document that prehistoric fi res occurred
mostly during the dormant season (starting as early as August and ending in
October) in both pine dominated and mixed conifer forests (Taylor 2000;
Beaty and Taylor 2001). In mixed conifer forests of the north-central, southcentral, and southern Sierra Nevada, fi res occurred most frequently just before
dormancy in latewood growth (Stephens and Collins 2004). If reintroducing
ecological processes is an important goal of a prescribed burn, it would be best
if the burns took place in a time consistent with the fi re history records.
Managers must consider many facets when choosing a location for treatment. With the amount of land rated at high hazard in California it would
be wise to target stands which would benefit the most from treatment. If
reintroduction of fi re into the ecosystem is the primary goal and fuel reduction the secondary goal, then choosing treatment locations could include
both stands with high and low fi re hazards. Unfortunately, there is no one
size fits all for fuel treatments in California; managers must consider many
factors when implementing a forest restoration plan.
474
USDA Forest Service Proceedings RMRS-P-41. 2006.
Effectiveness of Prescribed Fire as a Fuel Treatment in Californian Coniferous Forests
Vaillant, Fites-Kaufman, Stephens
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